(1) Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system that includes an intermediate transfer belt.
(2) Related Art
In recent years, as a full-color image forming apparatus, there has been widely used an image forming apparatus employing a so-called intermediate transfer system in which toner images of respective colors formed on a photosensitive drum are primarily transferred onto an intermediate transfer belt, respectively, and then the toner images overlaid on the intermediate transfer belt are secondarily transferred onto a recording sheet collectively.
According to such an image forming apparatus, primary transfer of the toner images formed on the photosensitive drum onto the intermediate transfer belt is performed in the following manner. A transfer roller is provided at a position substantially opposing the photosensitive drum with the intermediate transfer belt sandwiched between the transfer roller and the photosensitive drum. The transfer roller is pressed to an inner circumferential surface of the intermediate transfer belt. While a surface of the intermediate transfer belt is brought in contact with the photosensitive drum at an appropriate pressure, a predetermined transfer voltage is applied to the transfer roller to cause the toner images formed on the photosensitive drum to be electrostatically adsorbed to the intermediate transfer belt.
The transfer efficiency decreases due to each of a too high contact pressure and a too low contact pressure between the photosensitive drum and the intermediate transfer belt. Accordingly, it is necessary to set the contact pressure with a high accuracy.
On the other hand, there has been recently used an image forming apparatus in which a relative position is slightly offset between a transfer roller and a photosensitive drum in a running direction of an intermediate transfer belt, and a long distance is set between a first contact position where the transfer roller contacts with the intermediate transfer belt and a second contact position where the photosensitive drum contacts with the intermediate transfer belt.
There is a recent tendency that such an image forming apparatus adopts the structure in which the resistance value between the first contact position and the second contact position is increased to increase the voltage difference between the transfer roller and the photosensitive drum, thereby to increase the electrical field intensity and keep a high transfer efficiency (hereinafter, “offset type structure”).
According to an image forming apparatus having the offset-type structure, as shown in FIG. 12A, at each end of the transfer roller 1035, a disk-shaped rollers 1034 each having an outer diameter greater than an outer diameter of the transfer roller 1035 is provided coaxially with an axis of the transfer roller 1035. The rollers 1034 each abut with a non-image region located at each end of an outer circumferential surface of a photosensitive drum 1031.
In this way, the image forming apparatus having the offset-type structure keeps a constant distance between the photosensitive drum 1031 and the transfer rollers 1035 and keeps a constant value of a press stroke L11 of an intermediate transfer belt 1011, thereby to ensure the contact pressure with a high accuracy.
Here, the photosensitive drum 1031 is pivotally supported by a pair of first holding members 1132.
On the other hand, the transfer roller 1035 and the rollers 1034 are each pivotally supported at ends of a pair of second holding members 1036 that swing around a swing shaft 1036a. 
The second holding members 1036 are forced toward the photosensitive drum 1031 by a spring (not shown) or the like.
With such a structure, the rollers 1034 abut with the non-image regions located at the both ends of the outer circumferential surface of the photosensitive drum 1031.
The photosensitive drum 1031 is generally structured so as to be removable in consideration of maintenance and the like.
Accordingly, there are variations in the accuracy of assembling and the dimensional accuracy of components, for example. This tends to cause a relative positional offset between the first holding members 1132 holding the photosensitive drum 1031 and the second holding members 1036 holding the transfer roller 1035 and the rollers 1034.
For example, as shown in FIG. 12B, in the case where an offset occurs between the rotation axis 1031a and the swing shaft 1036a in the horizontal direction (X-axis direction) and the position of the swing shaft 1036a shifts from P1 to P1′, the abutting position where the rollers 1034 abut with the photosensitive drum 1031 shifts from P3 to P4 and the value of the press stroke of the intermediate transfer belt 1011 applied by the transfer roller 1035 increases from L11 to L12.
Also, as shown in FIG. 12C, in the case where an offset occurs between the rotation axis 1031a and the swing shaft 1036a in the perpendicular direction (Z-axis direction) and the position of the swing shaft 1036a shifts from P1 to P1″, the abutting position where the rollers 1034 abut with the photosensitive drum 1031 shifts from P3 to P5 and the value of the press stroke of the intermediate transfer belt 1011 applied by the transfer roller 1035 decreases from L11 to L13.
In this way, in the case where a relative positional offset occurs between the first holding members 1132 and the second holding members 1036, the press stroke of the intermediate transfer belt 1011 varies. This makes it difficult to set the contact pressure between the photosensitive drum and the intermediate transfer belt within an appropriate range. Accordingly, the transfer efficiency might decrease.